A known approach to three-dimensional vision uses the “time-of-flight” measurement principle. The range (distance) from the imaging apparatus to a point in the scene is measured by timing of the return of a pulse of light projected onto that point. Lidar systems transmit a modulated light beam into the scene and compare the modulation of the transmitted and returned signal to determine range. For instance, coherent frequency modulator (FM) lidar incorporates frequency modulation/mixing to determine range. However, FM lidar systems suffer from the limited frequency modulation capabilities of current laser diodes. Phase shift (AM) lidar is a preferred approach that correlates the phase shift between outgoing and incoming amplitude modulated light beams to determine range.
Lidar 3D imaging systems can be classified into two categories: Lidar imagers of the first category use refractive or reflective imaging optics to image the scene onto a detector array.
Lidar imagers of the second category obtain a range image of the scene in a time-division-multiplexed manner by scanning a light beam across the scene and determining a range value for each point illuminated by the light beam. To sweep the light beam through the scene to be imaged, one conventionally uses a scanning mirror. Document EP 0 448 111 discloses a lidar scanning system with a rotating multifaceted polygon mirror for transmitting modulated light from one of its facets to a surface to be imaged. Diffuse light reflected off the surface is received by another facet of the polygon mirror and reflected to a photo detector. The modulation phase difference between the transmitted and received light is then used to compute the range of the surface from the scanning system.
Current systems of both categories typically suffer from the disadvantage that they are cannot easily be reconfigured. Once the system has been set up, the field of view is usually fixed and can only be changed by replacing the imaging optics or the mechanical scanning device. Systems with automatic zooming are very complex and thus expensive. Furthermore, such systems typically comprise a high number of moving components, which is undesired for some applications. With scanning lidar imagers, the frame rate is furthermore given by the frequency of the scanning mirror.